The Eph family of receptor tyrosine kinases and their ligands, the ephrins, regulate cell movement and neuronal axon guidance during development, contributing to the establishment of the embryonic body plan. Eph/ephrin signaling pathways, however, are poorly understood. A unique feature is that binding of Eph receptors to transmembrane ephrins activates signals mediated by the ephrin cytoplasmic domain. EphB2 with ephrin-B1 results in tyrosine phosphorylation of the cytoplasmic ligands. Interaction of EphB2 with ephrin-B1 results in tyrosine phosphorylation of the cytoplasmic domains of both proteins. Effector proteins that bind to distinct tyrosine phosphorylated sequences propagate EphB2/ephrin-B1 signals by activating cytoplasmic signaling cascades. Ultimately, the signaling pathways activated by the EphB2/ephrin-B1 complex cause changes in cytoskeletal organization, cell-cell adhesion, and cell-matrix adhesion. Our goal is to characterize at the molecular level the signaling pathways activated by EphB2 and ephrin-B, and understand how they control the behavior of both EphB2 expressing cells and ephrin-B1 expressing cells. This will be accomplished by (i) mapping the tyrosine- phosphorylated sequence motifs that are created upon formation of the EphB2/ephrin-B1 complex; (ii) identifying signaling effectors that interact with the EphB2/ephrin-B1 complex; and (iii) dissecting EphB2 and ephrin-B1 signaling pathways that underlie different biological motifs of EphB2 and ephrin-B1 to specific biological activities, and contribute to elucidate how these Eph family proteins govern tissue patterning. These studies will also provide the knowledge necessary for modulating EphB2/ephrin-B1 signals when their malfunction causes disease. Aberrant EphB2/ephrin-B1 signaling could play a role in congenital neurological disorders and organ malformations, as well as other diseases involving tissue disorganization, including malignant tumor progression.